Abstract:

Hard cheese from yak milk was made with three starter cultures, namely mesophilic starter culture, thermophilic
starter culture and their 1:1 mixture. In the present study, we explored the relationship between bitter taste and proteolysis
of yak milk-derived hard cheeses during the maturation process of 6 months. The contents of pH 4.6-soluble nitrogen (SN),
12% trichloroacrtic acid (TCA)-N and free amino acids (FAAs), and the ratio of hydrophobic peptide to hydrophilic peptide
were determined. We found that the contents of pH 4.6-SN, 12% TCA-N and FAAs in the hard cheese, irrespective of the
starter culture used, increased during the maturation process. Bitterness value had a positive correlation with the contents of
pH 4.6-SN, 12%TCA-N and FAAs with correlation coefficients of 0.400, 0.412 and 0.458, respectively. The change trend
and degree of the ratio between hydrophobic peptide and hydrophilic peptide in the three hard cheeses from different starter
cultures were different during the maturation process. The ratio between hydrophobic peptide and hydrophilic peptide tended
to decline in the cheese made with mesophilic starter culture, but decreased first and then increased in the cheeses obtained
from thermophilic starter culture and mixed starter culture; the degree of change was different in the latter two cheeses. A
slight reduction in the ratio between hydrophobic peptide and hydrophilic peptide was observed for cheeses fermented by
thermophilic starter culture when the maturation period was extended from 1 month to 3 months, whereas a significantly
increased value was achieved by further prolonging the maturation period to 6 months. Nevertheless, the opposite results
were found for cheeses fermented by mixed starter culture. Bitterness value had a positive correlation with the ratio between
hydrophobic peptide and hydrophilic peptide (r = 0.895). The ratio between hydrophobic peptide and hydrophilic peptide co
uld reflect the intensity of cheese bitter. Bitterness was closely related to proteolysis. Starter culture with a stronger ability to
degrade milk protein easyly produced the bitter taste. Thermophilic starter culture had the strongest ability to degrade milk
protein, followed by mixed starter culture, and mesophilic starter culture.

Key words: bitterness value, yak milk, hard cheese, proteolysis, hydrophobic peptide